1. Field of the Invention
The present invention relates to an improved electron gun, and more particularly, to an advanced center post (ACP) gun for producing a hollow electron beam having either a small orbit or a large orbit.
2. Description of Related Art
It is well known in the art to utilize a linear beam device within a traveling wave tube (TWT), klystron, magnetron or other microwave device. In a linear beam device, an electron beam originating from an electron gun is caused to propagate through a tunnel or a drift tube generally containing an RF interaction structure. At the end of its travel, the electron beam is deposited within a collector or beam dump which effectively captures the spent electron beam. The beam is generally focused by magnetic or electrostatic fields in the interaction structure of the device in order for it to be effectively transported from the electron gun to the collector without loss to the interaction structure. An RF wave can be made to propagate through cavities within the interaction structure and interact with the electron beam which gives up energy to the propagating wave. Thus, the microwave device may be used an amplifier for increasing the power of a microwave signal.
The electron gun which forms the electron beam typically comprises a cathode and an anode. The cathode includes an internal heater which raises the temperature of the cathode surface to a level sufficient for thermionic electron emission to occur. When the potential of the anode is positive with respect to the cathode, electrons are drawn from the cathode surface and moved towards the anode. The geometry of the cathode and anode provide an electrostatic field shape which defines the electron flow pattern. The electronic flow then passes from the electron gun structure to the interaction region of the microwave device. An electron gun of this type is known as a Pierce gun.
In one particular type of Pierce gun, a hollow electron beam is formed. By varying the axial magnetic field, the electrons in the hollow beam can be made to orbit some of the magnetic flux lines. As the magnetic field is increased, a significant fraction of the axial energy of the electron beam is converted to motion transverse to the beam axis. This gyrating beam is used in several microwave devices which convert the transverse energy of the beam into RF energy. Examples of these devices are the peniotron, gyrotron, gyroBWO, gyroTWT, etc. A prior art gyrotron is shown in FIG. 1.
The cathode of a hollow beam gun is generally annular so that it emits a circular beam of electrons 18, as shown in FIG. 2a. The hollow beam can be characterized as either a large orbit beam in which the electrons 44 spiral about a guiding center of the beam near the axis of the microwave device in a circular path 42, or a small orbit beam in which the electrons orbit around individual flux lines of the guiding magnetic field in the interaction region. The rotation of the electrons in a large orbit beam is induced by a magnetic field reversal at the front end of the interaction region. The large and small orbit beams are shown graphically at FIGS. 2a and 3, respectively.
One class of devices utilize large orbit beams for production of a microwave output through a process known as cyclotron resonance maser (CRM) interaction. Maser is an acronym for microwave amplification using stimulated emission of radiation. CRM interaction devices extract rotational energy from the beam in radial cavities disposed within the interaction region. The electrons 44 orbit about the guiding center at a rate known as the cyclotron frequency .OMEGA..sub.c. The space charge forces within the gyrating electron beam result in azimuthal bunching 46 of the orbiting electrons, shown graphically in FIG. 2b. If the frequency of the propagating RF wave is slightly greater than the cyclotron frequency, the electron bunches fall back into a decelerating field and transfer their energy to that field. Interaction can also take place at harmonics of the cyclotron frequency. In this case, multiple bunches are formed equally spaced about the cyclotron orbit.
Both the efficiency and stability of CRM devices and peniotrons are strongly dependant on the ratio of transverse velocity to axial velocity of the beam, known as .alpha.. In these devices, the .alpha. value is usually between 1 and 2. Increasing .alpha.will raise the efficiency of these devices until the device becomes unstable. In order to obtain maximum efficiency of energy transfer to the RF wave, uniform transverse and axial velocity of the orbiting electrons is desired.
In practice, such velocity uniformity is difficult to achieve. A standard magnetron injection gun (MIG) has a conical cathode which produces a small orbit beam that is constrained to move axially by the applied magnetic field. In the standard MIG gun, magnetic flux threads the cathode in order to control the beam radius and improve beam stability. However, this type of MIG gun is impractical for producing a large orbit beam since the variation in flux across the cathode surface translates to variation in angular velocity after the magnetic field reversal. Other electron gun designs utilize a shielded cathode with a center post to reduce or eliminate the magnetic field at the cathode and decreases the transverse velocity spread. However, the beam radius of these guns is typically limited to the cathode radius, and can not be readily adjusted to accommodate very short wavelength RF signals, such as in the millimeter wavelength range. Thus, these shielded cathode designs have not been successfully applied in forming large orbit axis encircled beams in these applications.